This could be several things:
- The importer is outdated
- The gap between HEAD and the latest stable released version
- The dumps have some kind of incompatibility
- The mediawiki code is old/not suitable for Wikipedias
- The MySQL databases in production are not in sync with Mediawiki
Investigate and solve the issue, in case it is not already reported.
This is the link to the patch suggested on that article: https://gist.github.com/infusion/3c5007c73410b3fea3de76a10628c31e